Hi Tim,

I am not sure about potatoes but I had grown onions quite successfully from
seeds. I got those seeds from cheap chain shops. However, you can find
there are at least 4 varieties of onion seeds from Thompson & Morgan which
accept international order. There are more choice from 'the organic
gardening catalogue'.
http://www.thompson-morgan.com/
http://www.organiccatalog.com/catalog/

You can post your question in the 'guestbook' of T&M, usually you will get
your answer if you enquire about buying suitable seeds from the company.

Hope it can help.
CK